1. Field of the Invention
The present invention relates to storage devices for episodic signals, such as electrocardiogram (ECG) signals. More particularly, the present invention relates to storage of ECG signals on portable storage media, which is especially useful for monitoring of ambulatory patients, i.e. a "Holter" type monitors.
2. The Prior Art
Recording of episodic biological signals is known in the art. Such signals, for example ECG signals, have been observed as oscilloscope traces, have been stored in the form of paper chart records, and have been recorded on magnetic tape. It has been customary to record ECG signals on an analog magnetic tape recorder, generally using a direct recording technique. Occasionally, FM, pulse width, or pulse ratio modulation has been used. Today, it is common to record two separate ECG leads for twenty-four hours with a bandwidth that extends from just over 0.05 Hz to an upper limit between 50 and 100 Hz.
Direct digital storage is desirable to achieve lower weight and higher reliability than is possible with mechanical tape recording components. Semiconductor memory is available that would, in theory, permit such storage. Unfortunately, the present state of the art of semiconductor memory is such that memory chips are relatively expensive, take up a fair amount of circuit board space, and consume a fair amount of power. It is thus highly desireable to limit, where possible, the number of memory components in a system.
This present limitation of semiconductor memory presents a potential memory size problem. It has been experimentally determined that in order to digitally store an ECG signal with the fidelity necessary for accurate processing and visual reconstruction, it is necessary to sample the ECG at a rate of at least 125 samples per second. By performing some "automatic gain control," data may be stored in a processing computer as one byte per data point. If the data were stored directly at a rate of 125 samples per second, it would require 21.6 megabytes of storage to store two channels of ECG for twenty-four hours.
While there are semiconductor memory chips available today with densities as great as one megabit per chip, it is apparent that direct recording of the ECG signal for twenty-four hours at the rate of 125 samples per second would require the use of one hundred seventy two separate one megabit memory chips. From the standpoints of cost, power consumption and physical size, this requirement renders direct digital storage somewhat impractical for portable devices, unless some sort of data compression is employed.
In the literature, there is significant reference to a variety of algorithms for compression of the ECG data to minimize the amount of data to be stored. The most prominent among these are, Cox, J. R., Nolle, F. M., Fozzard, H. A. and Oliver, G. C., Jr. "AZTEC-A Preprocessing Program For Real Time ECG Analysis," IEEE Trans on Bio-Med Eng. (Short Commun.), Vol. BME-15, pp 128-129, April 1968; Mueller, William C., "Arrythmia Detection Program For An Ambulatory ECG Monitor," BIOMEDICAL SCIENCES INSTRUMENTATION Vol 14, PAGE 81, 1978; Hambly, Alan R., Moruzzi, Romeo L. and Feldmen, Charles L. "The Use Of Intrinsic Components In An ECG Filter," IEEE Trans On Bio-Med Eng. Vol. BME-21, No. 6 pp 469-476, November 1974.
While each of these data compression processes has some utility, each has inadequacies which make it unsuitable for the present application. There is thus a need for an apparatus which is capable of performing direct digital storage of the ECG signals and a method of storing these signals which provides suitable data compression for lowering the amount of memory required, while at the same time obtaining the capability to faithfully reproduce the important characteristics of an ECG signal.